Induction electric motors and generators typically include a stator and a rotor rotatable relative to the stator. The stator comprises a toroidal metal core or body and a plurality of winding coils made of conducting wire, typically copper or aluminum, arranged around the core and capable of generating an electromagnetic field when suitable currents flow therein. The winding coils are generally laced by means of a cord of natural, synthetic or metallic material, in order to improve the winding performance and to avoid possible interference with the moving rotor.
Several techniques and apparatuses for effecting the lacing of stator coils are known, for instance from U.S. Pat. No. 3,659,337, U.S. Pat. No. 3,862,493, U.S. Pat. No. 5,615,472 and U.S. Pat. No. 5,485,670. EP-A-1 081 831 in the name of the same applicant discloses a method and an apparatus for making a self-locking knot at the end of the winding lacing.
The known apparatuses employ a cord feeder associated with a needle, the feeder performing a vertical up and down movement (relative to a stator arranged with vertical axis) and an oscillating movement towards the needle's eye, while the needle hooking the cord and drawing it from the feeder performs both vertical (up and down) movements and radial movements by going into and out of the region defined by the circular perimeter of the stator (more precisely of the coil).
The cord is captured by the open eye of the needle, i.e. by the hook-shaped needle portion (head). The needle, when leaving the region defined by the coil circular perimeter, is rotated in order it does not engage the cord chain of the previous loop, and in order to be positioned again with the hook in the proper orientation for the subsequent cycle.
The known apparatuses allow a satisfactory lacing only for very well formed stator windings, with rather closely angularly spaced slots, that is with a reduced slot pitch.
The stator winding may exhibit irregularities and deformations and, in the known lacing systems, the imperfections of the coil heads to be laced cause a forced deviation of the lacing cord that therefore leaves the area, defined at the initial set-up, foreseen for the engagement between the cord and the needle's eye. Consequently, a lacing stitch can be missed, with the consequent need to stop the apparatus, to make the operator intervene and to discard the stator (which will be recovered later on). This wastes considerable time and labor, which in turn increases the lacing cost.
Moreover, in the known lacing systems, in order to make a synchronized and unidirectional phasing of the various movements necessary to perform the lacing, the indexed rotational movement of the stator presenting the different slot intervals to the needle is unidirectional (either clockwise or counterclockwise, depending on the manufacturer's choices). This gives rise to problems of compatibility of use among apparatuses of different manufacturers.
Lastly, for design reasons, the slots may have a great angular spacing and, under those conditions, typical of the new generation brushless motors, the known methods experience difficulties in correctly performing the lacing.
In the known systems the physical set-up between the needle and the cord feeder is often complex and requires continuous and very precise adjustments to synchronize the meeting or hooking point between the cord feeder and the needle.